The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to devices for implementing magnetic resonance elastography (MRE).
The physician has many diagnostic tools at his or her disposal which enable detection and localization of diseased tissues. These include x-ray systems that measure and produce images indicative of the x-ray attenuation of the tissues and ultrasound systems that detect and produce images indicative of tissue echogenicity and the boundaries between structures of differing acoustic properties. Nuclear medicine produces images indicative of those tissues which absorb tracers injected into the patient, as do PET scanners and SPECT scanners. And finally, magnetic resonance imaging (MRI) systems produce images indicative of the magnetic properties of tissues. It is fortuitous that many diseased tissues are detected by the physical properties measured by these imaging modalities, but it should not be surprising that many diseases go undetected.
Historically, one of the physician's most valuable diagnostic tools is palpation. By palpating the patient a physician can feel differences in the compliance of tissues and detect the presence of tumors and other tissue abnormalities. Unfortunately, this valuable diagnostic tool is limited to those tissues and organs which the physician can feel, and many diseased internal organs go undiagnosed unless the disease happens to be detectable by one of the above imaging modalities. Tumors (e.g., of the liver) that are undetected by existing imaging modalities and cannot be reached for palpation through the patient's skin and musculature, are often detected by surgeons by direct palpation of the exposed organs at the time of surgery. Palpation is the most common means of detecting tumors of the prostate gland and the breast, but unfortunately, deeper portions of these structures are not accessible for such evaluation. An imaging system that extends the physician's ability to detect differences in tissue compliance throughout a patient's body would extend this valuable diagnostic tool.
It has been found that MR imaging can be enhanced when an oscillating stress is applied to the object being imaged in a method called MR elastography. The method requires that the oscillating stress produce shear waves that propagate through the organ or tissues to be imaged. These shear waves alter the phase of the MR signals, and from this the mechanical properties of the subject can be determined. In many applications, the production of shear waves in the tissues is merely a matter of physically vibrating the surface of the subject with an electromechanical device such as that disclosed in above-cited U.S. Pat. No. 5,592,085. For example, shear waves may be produced in the breast and prostate by direct contact with the oscillatory device. Also, with organs like the liver, the oscillatory force can be directly applied by means of an applicator that is inserted into the organ.
A number of driver devices have been developed to produce the oscillatory force needed to practice MRE. As disclosed in U.S. Pat. Nos. 5,977,770, 5,952,828, 6,037,774, and 6,486,669, these typically include a coil of wire through which an alternating current flows. This coil is oriented in the polarizing field of the MRI system such that it interacts with the polarizing field to produce an oscillating force. This force may be conveyed to the subject being imaged by any number of different mechanical arrangements. Such MRE drivers can produce large forces over large displacement, but they are constrained by the need to keep the coil properly aligned with respect to the polarizing magnetic field. In addition, the current flowing in the driver coil produces a magnetic field which can alter the magnetic fields during the magnetic resonance pulse sequence resulting in undesirable image artifacts.
Another approach is to employ piezoelectric drivers as disclosed in U.S. Pat. Nos. 5,606,971 and 5,810,731. Such drivers do not produce troublesome disturbances in the scanner magnetic fields when operated, but they are limited in the forces they can produce, particularly at larger displacements. Piezoelectric drivers can also be oriented in any direction since they are not dependent on the polarizing magnetic field direction for proper operation.
Yet another approach is to employ an acoustic driver system as described in U.S. Pat. Nos. 7,034,534 and 7,307,423. The system includes a remotely located active acoustic driver acoustically coupled to one or more passive acoustic drivers positioned on the subject being imaged. The active driver includes a loudspeaker cone coupled to a ported cover. The ported cover is constructed of a rigid material such as polycarbonate and has a thin, rectangular shape. Acoustic, or pressure, waves generated by the loudspeaker cone are directed to the passive driver via a tube. In response, shear waves are produced by the passive driver and projected into the subject being imaged. The passive driver and tube do not disturb the magnetic fields and may be oriented in any direction.
This acoustic driver system has been shown to reliably generate shear waves during an MR elastography examination to obtain shear stiffness images, or elastograms. However, the shear waves produced by the passive driver are not strong enough to produce high resolution elastograms. This is especially true, for example, when imaging organs or regions that are large or are located deeper within the body. In other words, when heavy loading of the passive driver is required, such as in MRE imaging of the liver, the resulting elastograms do not have a desired degree of clarity or resolution. Attempts were made to improve the performance of the prior art acoustic driver system by applying higher levels of electrical power and using speaker units with more powerful voice coil motors. However, these attempts did not yield a sufficient improvement over the existing active driver design.
It was determined that these improvements did not improve the performance of the acoustic driver system because loudspeaker cones are designed for driving pressure waves through a low impedance medium such as free air. Acoustically coupling the active driver to the passive driver necessarily seals air within the drivers, thereby creating a high impedance medium. As such, acoustic waves lose energy while traveling through the sealed system. Attempting to produce pressure waves at higher energy levels fails, primarily due to flexing of the speaker cone caused by the sealed nature of the acoustic driver system.